In transport vehicles such as automobiles, railcars, and aircrafts, resistance spot welding or riveting has been used for connecting metal members together. However, in recent years, a method of joining metal members together by utilizing frictional heat (friction stir spot welding method) has been drawing attention (see Patent Literature 1, for example).
In the friction stir spot welding method disclosed in Patent Literature 1, an apparatus includes: a substantially columnar pin member; a substantially cylindrical shoulder member including a hollow portion in which the pin member is to be inserted; and a clamp member. The pin member and the shoulder member stir part of workpieces, thereby joining the workpieces together,